The United States isn’t the only place to experience an off-season RSV surge. Australia, South Africa, Iceland and various European countries have also done so. In France, RSV arrived four months late – April instead of December – according to Jean-Sébastien Casalegno, doctor and virologist at the Institute of Infectious Agents of the Hospices Civils de Lyon and first author of a March preprint describing the epidemic.
There aren’t many models to indicate what might happen next. Will the RSV return this year and have a smaller, weaker season in its normal time slot? Will it slowly turn around the calendar until it comes back to its place? “The seasonality will probably come back after several seasons,” says Casalegno. “What’s complicated is next season, what’s going to happen.”
Viruses are seasonal for complicated reasons, not just because they have changing preferences for particular temperatures and humidity, but because winters tend to be the time people congregate indoors. But they are also seasonal because it takes time to build up a large enough number of vulnerable people – those who have not been previously exposed, or vaccinated if a vaccine exists – to provide a virus with enough territory to reproduce and transmit. copies of itself. to new hosts.
The way this group of “susceptible” develops is slightly different for each virus. For RSV, which typically cycles annually, younger children are most at risk. By school age, most children have acquired immunity to infections or repeated exposures that did not cause symptoms but allowed their immune systems to build up a defense.
EV-D68 is also seasonal, but in a more complicated way. First, its outbreaks occur in the summer, not in the winter. Second, as demonstrated in the first analysis of its seasonality, published in March in Science Translational Medicine, both the respiratory illness it causes and this flexible paralysis seems to recur every two years. This analysis revealed that the cycles are determined by climatic conditions, but also by the immune system: women who are exposed to EV-D68 during pregnancy transmit antibodies against it to their infants. Thus, during their first 6 months, babies are protected against the disease and become vulnerable as this passive immunity wanes. This later vulnerability, combined with seasonality, seems to lead to a slower accumulation of susceptible subjects.
The last outbreak of EV-D68 was supposed to occur last summer, in 2020. Just like RSV and the flu, it didn’t happen, and for similar reasons: to hide, to get away, to wash your hands. hands and stay at home protected from children who would have been vulnerable then. And as with RSV, no one is sure what will happen next.
“There’s nothing about enteroviruses that’ll make them love them for years, they don’t have a lucky number,” says Kevin Messacar, associate professor of pediatrics at the University of Colorado and Children’s Hospital from Colorado, who was a co-author of this March analysis. “The model for this whole family of viruses, which is well described, wouldn’t predict that we would wait until 2022 for an epidemic because we missed a cycle. It looks like we’re continually increasing the number of susceptible people who haven’t seen this virus. “
A national project he is involved in, he says, finds lower-than-normal levels of EV-D68 antibodies in pregnant women because they were not exposed to the virus last year and therefore cannot transmit the virus. protection. This could mean that every time EV-D68 recurs, more children could contract the virus or become sicker than they otherwise would, or catch it earlier in their lives, over the months. the most vulnerable in early childhood, when they would otherwise have been protected.
And then there’s the flu, always the most unpredictable of respiratory infections, as it constantly mutates to escape our immune defenses, periodically swaps its dominant strains for new ones, and sometimes kicks off years of mild and sometimes devastating illness. The flu is also the future infection that causes the most anxiety right now. Without a dramatic return to social distancing, “I expect an exceedingly bad flu season,” says Sarah Cobey, immunologist and associate professor of ecology and evolution at the University of Chicago. “I expect more people to get infected with the flu. I also expect there will be a lot of really serious flu infections. “